Add build script to include target-libdir on rpath

crates/rustc_plugin/examples/print-all-items/Cargo.toml

@@ -11,3 +11,6 @@ rustc_plugin = { path = "../.." }
 env_logger = { version = "0.10", default-features = false }
 clap = { version = "4.4", features = ["derive"] }
 serde = { version = "1", features = ["derive"] }
+
+[build-dependencies]
+rustc_plugin = { path = "../.." }

crates/rustc_plugin/examples/print-all-items/build.rs

@@ -0,0 +1,5 @@
+#![feature(rustc_private)]
+
+fn main() {
+  rustc_plugin::build_main();
+}

crates/rustc_plugin/src/build.rs

@@ -0,0 +1,31 @@
+use std::{
+  path::{Path, PathBuf},
+  process::Command,
+};
+
+fn rustc_path() -> PathBuf {
+  let output = Command::new("rustup")
+    .args(["which", "--toolchain", crate::CHANNEL, "rustc"])
+    .output()
+    .expect("failed to run rustup which");
+  let rustc_path = String::from_utf8(output.stdout).unwrap();
+  PathBuf::from(rustc_path.trim())
+}
+
+fn target_libdir(rustc: &Path) -> PathBuf {
+  let output = Command::new(rustc)
+    .args(["--print", "target-libdir"])
+    .output()
+    .expect("failed to run rustc --print target-libdir");
+  let libdir = String::from_utf8(output.stdout).unwrap();
+  PathBuf::from(libdir.trim())
+}
+
+pub fn build_main() {
+  let rustc_path = rustc_path();
+  let target_libdir = target_libdir(&rustc_path);
+  println!(
+    "cargo::rustc-link-arg=-Wl,-rpath,{}",
+    target_libdir.display()
+  );
+}

crates/rustc_plugin/src/cli.rs

@@ -26,7 +26,7 @@ pub fn cli_main<T: RustcPlugin>(plugin: T) {
     .other_options(["--all-features".to_string(), "--offline".to_string()])
     .exec()
     .unwrap();
-  let plugin_subdir = format!("plugin-{}", env!("RUSTC_CHANNEL"));
+  let plugin_subdir = format!("plugin-{}", crate::CHANNEL);
   let target_dir = metadata.target_directory.join(plugin_subdir);
 
   let args = plugin.args(&target_dir);

crates/rustc_plugin/src/lib.rs

@@ -9,12 +9,19 @@ extern crate rustc_driver;
 extern crate rustc_interface;
 extern crate rustc_session;
 
+pub use build::build_main;
 #[doc(hidden)]
 pub use cargo_metadata::camino::Utf8Path;
 pub use cli::cli_main;
 pub use driver::driver_main;
 pub use plugin::{CrateFilter, RustcPlugin, RustcPluginArgs};
 
+/// The toolchain channel that this version of rustc_plugin was built with.
+///
+/// For example, `nightly-2025-08-20`
+pub const CHANNEL: &str = env!("RUSTC_CHANNEL");
+
+mod build;
 mod cli;
 mod driver;
 mod plugin;